via Compendium:
Fainting spell rulings:
Fainting Spell
* Gengar is receiving errata for its Poke-POWER, "Fainting Spell". Fainting Spell affects your opponent's Attacking Pokemon, not your Defending Pokemon. The correct card text should read, "Once during your opponent's turn, if Gengar would be Knocked Out by damage from an attack, you may flip a coin. If heads, the Attacking Pokémon is Knocked Out." (Nov 7, 2008 Pokemon Organized Play News)
Q. Gengar's "Fainting Spell" Poke-POWER says when it's KO'd by damage from an opponent's attack to flip a coin and if heads "the Defending Pokemon is Knocked Out". But since Gengar was acting as the "Defending Pokemon" when it was attacked what does this mean?
A. To clarify, if the coin flip is heads, Fainting Spell causes the opponent's Attacking Pokemon, which KO'd Gengar, to be Knocked Out. (Oct 30, 2008 PUI Rules Team)
Q. If I use Uxie's "Psychic Restore" attack where it does 20 and puts itself into the deck, but Gengar's "Fainting Spell" Poke-POWER would knock out Uxie, what happens?
A. If you choose to put Uxie in the deck with Psychic Restore the attack completes first, and Gengar cannot return the knockout. However, if you do not put Uxie in the deck, then Gengar's Fainting Spell would Knock Out Uxie if the coin flip is heads. (Oct 30, 2008 PUI Rules Team)
Q. If Gengar is KO'd by a Pokemon using some kind of hit & run attack, does Gengar's "Fainting Spell" Poke-POWER target the new Defending Pokemon or the one that just attacked?
A. The one that attacked and went to the bench would be KO'd if Gengar flips heads. (Oct 30, 2008 PUI Rules Team)
Q. What happens if I Knock Out my opponent's Gengar using some kind of hit & run attack, but I bring up Glaceon Lv.X with the "Chilly Breath" Poke-BODY that shuts down Poke-Powers?
A. The Chilly Breath would shut off Gengar's Fainting Spell before it would have a chance to be used. (Jan 15, 2009 PUI Rules Team)